How to Calculate Your Due Date: Naegele's Rule, Trimesters and the 280-Day Count

A positive test answers one question and immediately asks another: when is this baby due? The answer runs almost everything that follows — when the first scan is booked, when you tell your employer, when leave starts — and it comes from a single date most people have to think hard to remember. This guide explains how a due date is calculated with Naegele's rule, works a complete example from last period to trimester dates, and shows where the estimate can mislead — with our pregnancy due date calculator doing the calendar arithmetic at each step.

How to Calculate Your Due Date, Step by Step

1. Find the first day of your last menstrual period. Not the day it ended and not the day you expected the next one — the day bleeding began. A period-tracking app, a calendar note or a pharmacy receipt often pins it down. 2. Add 280 days. That is Naegele's rule: due date = first day of last period + 280 days, or exactly 40 weeks. Counting 280 days by hand across month ends and leap years is exactly the kind of arithmetic calendars punish, which is why the calculator exists. 3. Read off the supporting dates. The same calculation returns your likely conception date — about 14 days after the period started, around ovulation in a typical 28-day cycle — and the exact calendar span of each trimester. 4. Note how far along you are today. If the period date is in the past, the calculator also gives your current week and day, the number every appointment and milestone is quoted in. Then book the first antenatal check-up: the calculator plans dates, it does not provide medical care, and an early dating scan can refine everything below.

Worked Example: A Last Period Starting 1 January 2026

Take a last menstrual period that began on 1 January 2026. Adding 280 days lands on 8 October 2026 — the estimated due date. The likely conception date is 1 January plus 14 days: 15 January 2026. The trimesters fall out of the same count: the first runs from 1 January to 8 April 2026 (weeks 0 to 13+6), the second from 9 April to 15 July 2026 (weeks 14 to 27+6), and the third from 16 July through to the due date on 8 October 2026 (weeks 28 to 40). Notice what the example quietly demonstrates: on the day of conception, 15 January, this pregnancy was already “two weeks along,” because the 40-week clock started at the period. And hold the due date itself loosely — only about 5% of babies arrive on it. Think of 8 October as the centre of a four-week window, 37 to 42 weeks, inside which a normal arrival is expected.

What the First Trimester Dates Are Actually For

The trimester dates are not trivia; they are a schedule. The first trimester is when the most dramatic development happens in the smallest window: around week 4 the missed period turns a home test positive, by week 5 the neural tube that becomes the brain and spine is forming — which is why folic acid matters most right at the start — a heartbeat may be visible on ultrasound around week 6, all major organs have begun forming by week 8, and by week 12, at roughly 5–6 cm, the first trimester closes and miscarriage risk drops sharply. Those weeks are also when the practical appointments cluster: the first antenatal check-up, the dating scan that can confirm or shift the calculator's estimate, and the start of folic acid and the end of alcohol and smoking if those are still open questions. Knowing your week number early — the calculator's weeks-and-days figure — is what lets you book each of those into the right week instead of discovering them late. Once the baby arrives, the same date arithmetic runs in reverse: parents reach for our age calculator to work out exact age in weeks and months for vaccinations and milestones.

When the 280-Day Count Misleads

Naegele's rule is a convention built on a textbook cycle: 28 days, ovulation on day 14. Real cycles vary, and every day your ovulation sits away from day 14 moves the true due date by roughly a day. A 35-day cycle ovulates about a week later, so the period-based date can promise the baby a week early; a 24-day cycle does the opposite. Irregular cycles widen the error further, and the rule cannot account for assisted reproduction — where the conception date is known exactly and different dating is used — or for multiple pregnancies, which often deliver earlier. This is precisely why the first-trimester ultrasound is the medical standard: it measures the pregnancy itself rather than inferring it from a calendar assumption, and doctors will move the official date when the scan disagrees. Use the calculator's date to plan, book and count down — and your clinician's date, once you have it, to decide. General health context belongs in the same spirit: our BMR calculator guide explains how calorie needs are estimated, though pregnancy targets specifically are a conversation for your own care team, not a formula.

Common Mistakes

Counting from the wrong day. The rule starts on the first day of the last period; using the last day, or the day the next period was due, shifts everything by days or weeks. Treating the due date as an appointment. It is the middle of a 37-to-42-week window — about 1 in 20 babies arrives on the day itself, so plan leave and help around the window, not the date. Ignoring your cycle length. A neat 280-day answer feels precise even when a long or irregular cycle has already moved the real date by a week; flag your pattern at the first visit. Skipping the dating scan because the maths looked certain. The scan exists to correct the calendar, and it is most accurate in the first trimester — the one appointment that should never slide. Using a calculator as medical care. It dates a pregnancy for planning; it does not diagnose, monitor or advise. Anything worrying — bleeding, pain, reduced movement later on — goes straight to a clinician, not a website.

Frequently Asked Questions

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